LeetCode Reorder List

Q:

Given a singly linked list L: L0→L1→…→Ln-1→Ln,

reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,

Given

{1,2,3,4}

, reorder it to

{1,4,2,3}

.

题意很简单，就是将链表节点顺序按要求改变一下，但是不能单独改变节点的值，也就是说ListNode的val是不能改变的，只有next能改。开始想这还不简单，直接两个循环！结果当然超时。其实可以将这个链表用快慢指针分成两段，将后面一段反转，再依次插入到前面的链表中。

代码：

class Solution {
public:
ListNode * reverse(ListNode *head){
ListNode *cur = head, *pre = NULL, *next = NULL;
while(cur)
{
next = cur->next;
cur->next = pre;
pre = cur;
cur = next;
}
return pre;
}

void reorderList(ListNode *head) {
if(!head)return;
ListNode *temp = head;
ListNode *p1 = head;
ListNode *p2 = head;
while(p2)
{
temp = p1;
p1 = p1->next;
p2 = p2->next;
if(p2)
p2 = p2->next;
}
temp->next = NULL;
p2 = reverse(p1);
p1 = head;
ListNode * p = p1;
while(p1 && p2)
{
ListNode * curNext = p1->next;
p1->next = p2;
p2 = p2->next;
p1->next->next = curNext;
p = p1;
p1 = curNext;
}
if(p2)p->next->next = p2;
}
};